The present invention relates to an arrangement for transmitting radio frequency signal, i.e. RF signal, at two different transmitting frequency bands and for receiving at two different reception frequency bands. The arrangement is applicable, among others, in a transceiver of a radio communications system operating at two frequency bands or in a transceiver meant to be used in connection with two radio communications systems.
Mobile communications systems develop and grow rapidly, wherefore in many areas, there have been built or are being built systems according to several different standards. Consequently a need has arisen for such mobile stations that can be used in more than one system. Moreover, there are being developed new, so-called third-generation systems that will probably require dual mode operation of the receiver. Among these systems, let us point out the UMTS (Universal Mobile Telecommunications System) defined by the ETSI (European Telecommunications Standards Institute) and the FPLMTS (Future Public Land Mobile Telecommunications Systems) defined by the Radio Sector of the International Telecommunication Union.
FIG. 1 is a block diagram of the radio frequency parts, i.e. RF parts, of a prior art multi-mode mobile telephone operating at two different frequency bands. This type of arrangement is described in the patent application [1] EP 678,974 A2. The system includes a separate RF front end for both frequency bands as well as for the receiver and the transmitter respectively; in the receiver part, this RF front end comprises a pre-amplifier 2, 27 and an RF filter 3, 28, and in the transmitter part filters 19, 23 and a power amplifier 18, 24 as well as duplex filters 1, 25 for both frequency bands.
In order to be able to use the respective RF parts at each frequency band, two-way switches 26, 29, 31 are provided in the antenna front end and at the antenna-side ports of the mixers of the receiver and transmitter. While operating at two different frequency bands, two duplex filters 1, 25, two pre-amplifiers 2, 27 and two RF filters 3, 28 are employed in the signal reception. Respectively, in signal transmission, there are used two RF filters 19, 23 of the transmitter, two power amplifiers 18, 24 and two duplex filters 1, 25. By means of the two-way switches 26, 29, 31, one of the two blocks of the same type are always in use. In order to create two different first mixer frequencies LO1, in the synthesiser S1 there are provided two voltage controlled oscillators 13, 30, one of which is switched to be active by the controls V1 and V2, depending on the respective frequency band of operation, for instance the frequency band of the GSM or the PCN system. The employed controls VI and V2 can be the oscillator operating voltages (i.e. one is switched to operating voltage, the other is not). Each oscillator 13, 30 gives a different output frequency. Instead of different oscillators, in the formation of different mixer frequencies LO1 it could be possible to use two different frequency synthesizers S1, one of which is always selected to use, depending on the frequency band in question. Yet another alternative is to use one synthesiser S1, the phase-locked loop 15 whereof includes two different frequency dividers, one of which is always eligible for use. The division number of the divider 22 is chosen by the control NX. The division number for the PCN system is 2 and for the GSM system 1.
From the mixer 4, the signal first passes through a first intermediate frequency filter 5 to a second mixer 6, where it is mixed with the second mixer frequency LO2 in order to create a second intermediate frequency IF2. From the second mixer 6, the signal passes through a second intermediate frequency filter 8 and through +45.degree. and -45.degree. phase transfer blocks 10 and 11 to a demodulator 12, advantageously to an I/Q demodulator, where the local oscillator frequency LO3 also is brought.
In respective fashion, in the transmitter the signal I-TX, Q-TX entering a modulator in order to be transmitted is taken from the modulator 21, advantageously an I/Q modulator--where in addition to the signal to be transmitted, there also is brought as the modulator carrier wave signal, the frequency of the second mixer frequency LO2 divided by the divider 22--into a mixer 20; in said mixer 20, the signal is mixed to transmitting frequency FTX with the first mixer frequency LO1. From the mixer 20, the signal is brought through a transmitter-frequency RF filter 19 to a power amplifier 18, wherefrom the amplified signal is brought via the duplex filter 1 to the antenna ANT.
In the description the GSM and PCN systems are as examples: the operating frequencies used in said systems are as follows:
______________________________________ GSM: reception: 935.2 . . . 959.8 MHz transmission: 890 . . . 915 MHz mixer frequency LO1: 1215.6 . . . 1240.2 MHz (RX) 1222.6 . . . 1247.2 MHz (TX) mixer frequency LO2: 332.4 MHz frequency LO3: 26 MHz PCN: reception: 1805.2 . . . 1879.8 MHz transmission: 1710.2 . . . 1784.8 MHz mixer frequency LO1: 1524.8 . . . 1599.4 MHz (RX) 1544.0 . . . 1618.6 MHz (TX) mixer frequency LO2: 332.4 MHz local oscillator frequency 26 MHz LO3 of the demodulator: ______________________________________
A drawback of the prior art solution according to FIG. 1 is that for its RF interface, there is needed a remarkable number of filters (1, 3, 19, 23, 25, 28) and controllable switches (26, 29, 31) on the signal path, which makes the RF front end complicated and increases production expenses. Moreover, the switches cause attenuation on the signal path in the conducting mode, and "leak", i.e. pass signals also in the open state.